1. Field of the Invention
This invention relates to systems for synchronizing the operation of electronic circuits, and in particular to a voltage controlled variable delay line phase-locked loop system for synchronizing the operation of a central processing unit and a floating point coprocessor.
b. 2. Description of the Prior Art
The synchronization of several circuits operating in a system is a problem faced by many system designers. For example, in high performance microprocessor-based systems, it is desirable to synchronize the operation of the various individual integrated circuits within the system to minimize dead time on the system bus, as well as prevent undesired data collisions. In automatic test systems, to assure validity for the input and output signals from the device being tested, all of the input pins of the device must be driven essentially simultaneously, while the condition of the output pins is monitored.
One solution to synchronizing microprocessor-based systems, not necessarily in the prior art, is described in "A 32-Bit VLSI CPU with 15-MIPS Peak Performance" by M. Forsyth, et al., IEEE Journal of Solid-State Circuits (Oct. 1987) SC-22(5):768-775. The system described in that paper employs a global master clock to which all integrated circuit operations are synchronized.
One well known technique for synchronizing the operation of the circuits is to employ a phase-locked loop circuit. The phase-locked loop circuit typically employs a phase detector to compare an input signal with a reference signal from a voltage-controlled oscillator. The output of the phase detector thus is proportional to the difference in phase between the input signal and the signal from the voltage-controlled oscillator. The output of the phase detector, representing the error between the signals, then is supplied through a low pass filter to control the voltage-controlled oscillator. The error signal causes the voltage-controlled oscillator to lock to the incoming signal, despite an initial tuning which might not correspond to the input signal.